Enhanced production of HD and D_2 molecules on small dust grains in diffuse clouds

Motivated by recent observations of deuterated molecules in the interstellar medium, we examine the production of HD and D${}_2$ molecules on dust grain surfaces. A mechanism for the enhanced production of these deuterated molecules is studied. This mechanism applies on grain surfaces on which D atoms stick more strongly than H atoms, under conditions of low flux and within a suitable range of temperatures. It is shown that under these conditions the production rates of HD and D${}_2$ are greatly enhanced (vs. the H${}_2$ production rate) compared with the expected rates based on the adsorption of gas-phase atomic abundances of D and H. The enhancement in the formation rate of HD is comparable with the enhancement due to gas-phase ion-molecule reactions in diffuse clouds.